A modified approach to the histologic diagnosis of onychomycosis.

BACKGROUND Histologic examination of nail clippings with periodic acid-Schiff staining is the most sensitive diagnostic test for onychomycosis; however, difficulties in processing nail plates limit its use. In onychomycosis, fungi are most concentrated in the subungual hyperkeratosis rather than in the nail plate. We hypothesized that the diagnosis of onychomycosis could be effectively made from histologic examination of subungual hyperkeratosis alone. Specimens of subungual hyperkeratosis, unlike nail plates, can be processed in the same routine manner as skin specimens, allowing for the diagnosis of onychomycosis to be made more quickly and at lower cost. OBJECTIVE We investigated whether the diagnosis of onychomycosis could be effectively made from histologic examination of subungual hyperkeratosis alone. METHODS We selected all nail specimens submitted during an 8-month period to the New York University Dermatopathology Section for evaluation of onychomycosis that had subungual hyperkeratosis associated with the nail plate. Nail specimens were divided into two components: a subungual hyperkeratosis component and a nail plate component. The subungual hyperkeratosis was processed separately in a routine fashion and embedded in paraffin and examined. We determined the percentage of cases of onychomycosis in which hyphae were present in the subungual component. RESULTS Sixty-six cases of onychomycosis were diagnosed histologically during the study period. Ninety-seven percent of these cases had hyphae in the subungual component. In 3% of cases, hyphae were present in the nail plate component but not in the subungual component. LIMITATIONS This modified approach to diagnosing onychomycosis can only be utilized when an adequate amount of subungual hyperkeratosis is submitted. CONCLUSIONS The diagnosis of onychomycosis can be effectively made from histologic examination of subungual hyperkeratosis alone in most cases. This method circumvents the need to process nail plates in the vast majority of cases of onychomycosis (97%), resulting in a more efficient, less costly, and technically easier way of diagnosing onychomycosis. Submitting ample amounts of subungual hyperkeratosis is essential to increasing the diagnostic yield of nail clippings.